The invention relates to the field of dual flow turbojets fitted to subsonic aircraft and including a long pod terminated by a common nozzle for ejecting both the primary flow and the secondary flow.
More precisely, the invention relates to a turbojet of axis X housed completely inside a tubular pod whose inside wall co-operates with the case of said engine to define an annular passage for a secondary flow delivered by a fan, said pod having an air inlet upstream of the engine, thrust-reverser means in its middle section, and a common nozzle for ejecting both the primary flow and the secondary flow, the outlet of the nozzle being situated downstream from the engine, said thrust-reverser means being suitable for taking up an active position in which the secondary flow is diverted outwards and forwards from said pod, said nozzle having an enclosure defined by the inside and outside walls of said nozzle.
The downstream end of the engine case which lies between the hot primary flow stream and the cold secondary flow stream is fitted with a mixer for the purpose of accelerating mixing of the hot and cold flows so as to reduce the speed of the gases ejected via the common nozzle, thereby improving propulsive efficiency and reducing the noise of the jet.
Under cruising conditions or at full throttle, the thrust-reverser means are inoperative. The entire flow of air traveling in the secondary stream is ejected by the common nozzle together with the hot gases of the primary flow, and the inside wall of the nozzle is wiped by cold air. This mode of operation is referred to as xe2x80x9cforward jetxe2x80x9d mode.
In contrast, in xe2x80x9creversed jetxe2x80x9d mode, the thrust-reverser means shut off the secondary stream, and the secondary flow is diverted outwards and forwards from the pod, with only the hot gases of the primary stream passing through the ejection nozzle. The nozzle is then subjected to high temperatures. xe2x80x9cReversed jetxe2x80x9d mode is used only for braking the aircraft after it has landed, and in order to brake the aircraft effectively, it is necessary for the turbojet to deliver high power, thereby increasing the intensity and the temperature of the hot flow ejected by the nozzle. That is why the common nozzle must be made of a material that withstands the temperature of the hot flow on its own, such as titanium, thereby increasing both its mass and its cost.
U.S. Pat. No. 3,826,088 relates to a turbojet for military use, that is not fitted with thrust-reverser means. The nozzle of that turbojet surrounds a jacket for thermally protected a post-combustion chamber, and it co-operates therewith to define an annular channel that is continuously fed by a fraction of the cold flow. The jacket has slots that continuously deliver a film of air to its inside face. The jacket is clearly not a structural element of the nozzle since it must be capable of expanding freely as a function of temperature variations inside the post-combustion chamber.
FR 2 593 237 discloses a device for bleeding air from the cold flow of a dual flow engine with a thrust-reverser, the air that is bled off being used for cooling, for pressurizing, or for ventilating the airplane. However that document does not mention or suggest cooling the nozzle in xe2x80x9creversed jetxe2x80x9d mode of operation in a dual flow turbojet.
The object of the invention is to propose a turbojet as specified in the introduction in which the ejection nozzle can be made of a material of lower density.
According to the invention, the object is achieved by the fact that the proposed turbojet further comprises cooling means for cooling the nozzle when the thrust-reverser means are in the active position, said cooling means comprising means for bleeding a flow of cooling air from the secondary flow upstream from the thrust-reverser means, means for taking the bled-off flow of air into the enclosure of said nozzle, and means for forming a film of cooling air on the inside face of said nozzle.
The invention thus makes use of the pressure difference between the upstream and downstream sides of the thrust-reverser means in xe2x80x9creversed jetxe2x80x9d mode to drive a flow of air bled off into the cooling device. The higher pressure in the secondary flow upstream from the thrust-reverser means comes from the rotation of the fan and from the air which penetrates into the air inlet of the turbojet under the effect of the speed of the airplane. The lower pressure downstream from the thrust-reverser means comes from the suction effect in the common nozzle, likewise under the effect of the speed of the airplane.
Because the invention ensures the nozzle is cooled when in xe2x80x9creversed jetxe2x80x9d mode, the nozzle can be made out of a material that is of lower density and of reduced cost.
Advantageously, the means for forming a film of air on the inside face of said nozzle comprise multiple perforations formed through the inside wall of said nozzle.
In a first embodiment of the invention, the flow of cooling air is bled from the secondary flow by means of at least one scoop.
When the thrust-reverser means comprise at least one door mounted to tilt on longitudinal beams of the pod by means of support pivots, and in a first embodiment of the invention, the scoop is provided on the inside face of the door and the means for bringing the bled-off flow of air into the enclosure of the nozzle comprise a duct connecting the scoop to said enclosure via a bore provided axially in a support pivot for said door.
In a second embodiment of the invention, the scoop is provided on the inside face of a longitudinal beam upstream from the support pivot, and the means for bringing the bled-off flow of air into the enclosure of the nozzle comprise a duct connecting the scoop to said enclosure.
When the thrust-reverser means comprise fixed grids suitable for being masked by axially-movable shutters, and tilting flaps suitable for deflecting the secondary flow towards the grids when in the deployed position, the scoop is provided on the engine case upstream from the flaps in the deployed position, and the means for bringing the bled-off flow of air into the nozzle enclosure comprise a duct connecting the scoop to said enclosure via a radial arm connecting the nozzle to the engine case.
In any event, the shape and position of the scoop are designed so that the scoop does not excessively disturb the secondary flow in xe2x80x9cforward jetxe2x80x9d mode, and in such a manner that in xe2x80x9creversed jetxe2x80x9d mode the pressure of the air in the scoop is sufficient to ensure that air is bled-off at a flow rate that is sufficient to cool the nozzle.
In a second embodiment of the invention, the flow of cooling air is bled from the engine at a valve for regulating the device for controlling turbine clearance, and the means for bringing the bled-off flow of air into the enclosure of the nozzle comprise a duct connecting said valve to said enclosure via a radial arm connecting the nozzle to the engine case.